Accurate electro-optical displacement measurement of a single spatial dimension has been accomplished using interferometric and moire techniques.
Interferometric methods use the wavelength of a light source as a reference for measuring displacement, and typically require expensive precision optical components. Interference fringes are counted yielding a value representative of displacement.
Moire grating methods use the wavelength or pitch of optical gratings as their reference. Displacement is measured by detecting the relative position of two (more in some cases) such gratings. Typically, this is done using a relatively small number of photodetectors that detect two optical signals differing in phase by .pi./2 and modulated by the gratings. In a vernier-type moire method that allows for interpolation between moire fringes by eye, two gratings differing slightly in pitch are used. However, the resolution attainable by a small number of photodetectors is low. Also, the human eye is limited in its ability to read vernier scales.
A variation of the moire method is described in U.S. Pat. No. 4,529,964 granted on July 16, 1985 (Minami et al.). As disclosed in this patent, an array of photodetectors is used to detect an optical signal modulated by a single optical grating. (The optical grating can be considered as a main scale and the array of photodetectors as a vernier scale). From the optical data read by the array, a signal processing means determines an interpolated optical phase angle corresponding to interpolated displacement by following a `phase inverting` point. However, the precision of interpolation is limited to the data from only one data point out of the photodetector data array.
In interferometric surface profiling applications, Fourier analysis in both the temporal and spatial domains has been used to interpolate displacements perpendicular to the measuring device. Video tubes and charge coupled devices ("CCDs") are used to capture a matrix of displacement data which is used to analyze the surface under test. However, while these techniques work well to measure a 3 dimensional surface, the precision of each individual displacement measurement is not extreme. An example of spatial Fourier analysis applied to surface profiling can be found in Traditional and Synchronous Convolution Methods for Processing Fringe Pattern Images", Womack, K.H., Ph.D. dissertation, University of Arizona, 1982.
Transform methods such as the Fast Fourier Transform ("FFT") have been used to calculate phase. In "Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry", J Opt Soc. Am., Vol. 72, No. 1, January 1982, Takeda et al. discuss the use of the FFT for fringe pattern analysis.
An object of the present invention is to provide new and improved apparatus for measuring displacement.
A further object of the present invention is to provide new and improved opto-electronic apparatus capable of high resolution displacement measurements.